Bash String Comparison

When it comes to programming in Bash, string comparison is a fundamental concept that every developer needs to be familiar with. Bash string comparison involves comparing two strings and evaluating whether they are equal, not equal, greater than, or less than each other. Understanding how to compare strings in Bash is essential for writing reliable and robust scripts.

In this article, we'll explore basics of Bash string comparison and provide examples to help you understand concept better. We'll start with an introduction to comparison operators and then move on to different types of string comparison techniques.

Comparison Operators

Bash provides various operators to compare strings, including ==, !=, <, >, -z, and -n. Let's take a closer look at each of these operators.

= = Operator

The == operator checks if two strings are equal. Here's an example −

Example

string1="Hello"
string2="Hello"
if [ "$string1" == "$string2" ]
then
   echo "The two strings are equal"
fi

Output

The two strings are equal

In this example, we have two strings "Hello" stored in variables string1 and string2. == operator checks if two strings are equal, and if they are, program prints "The two strings are equal" on screen.

!= Operator

The != operator checks if two strings are not equal. Here's an example −

Example

string1="Hello"
string2="World"
if [ "$string1" != "$string2" ]
then
   echo "The two strings are not equal"
fi

Output

The two strings are equal

In this example, we have two strings "Hello" and "World" stored in variables string1 and string2. != operator checks if two strings are not equal, and if they are not, program prints "The two strings are not equal" on screen.

< Operator

The < operator checks if first string is less than second string. Here's an example −

string1="Apple"
string2="Banana"
if [ "$string1" < "$string2" ]
then
   echo "The first string is less than second string"
fi

In this example, we have two strings "Apple" and "Banana" stored in variables string1 and string2. < operator checks if first string "Apple" is less than second string "Banana", and if it is, program prints "The first string is less than second string" on screen.

> Operator

The > operator checks if first string is greater than second string. Here's an example −

string1="Banana"
string2="Apple"
if [ "$string1" > "$string2" ]
then
   echo "The first string is greater than second string"
fi

In this example, we have two strings "Banana" and "Apple" stored in variables string1 and string2. > operator checks if first string "Banana" is greater than second string "Apple", and if it is, program prints "The first string is greater than second string" on screen.

-z Operator

The -z operator checks if string is empty. Here's an example −

Example

string1=""
if [ -z "$string1" ]
then
   echo "The string is empty"
fi

Output

The string is empty

In this example, we have an empty string stored in variable string1. -z operator checks if string is empty, and if it is, program prints "The string is empty" on screen.

-n Operator

The -n operator checks if string is not empty. Here's an example −

Example

string1="Hello"
if [ -n "$string1" ]
then
   echo "The string is not empty"
fi

Output

The string is not empty

In this example, we have a non-empty string "Hello" stored in variable string1. -n operator checks if string is not empty, and if it is not, program prints "The string is not empty" on screen.

String Comparison Techniques

Apart from comparison operators, there are several techniques for comparing strings in Bash. Let's explore a few of these techniques.

Numeric Comparison

Numeric comparison is used when comparing numbers stored as strings. For example −

Example

string1="10"
string2="20"
if [ "$string1" -lt "$string2" ]
then
   echo "The first number is less than second number"
fi

Output

The first number is less than second number

In this example, we have two numbers "10" and "20" stored in variables string1 and string2. We use -lt operator to check if first number is less than second number, and if it is, program prints "The first number is less than second number" on screen.

Case-Insensitive Comparison

Case-insensitive comparison is used when comparing strings regardless of their case. For example −

Example

string1="hello"
string2="HELLO"
if [ "${string1,,}" == "${string2,,}" ]
then
   echo "The two strings are equal"
fi

Output

The two strings are equal

In this example, we have two strings "hello" and "HELLO" stored in variables string1 and string2. We use ,, parameter expansion to convert both strings to lowercase and then use == operator to check if two strings are equal. If they are, program prints "The two strings are equal" on screen.

Substring Comparison

Substring comparison is used when comparing a part of a string. For example −

Example

string1="Hello World"
substring="Hello"
if [[ "$string1" == *"$substring"* ]]
then
   echo "The substring is present in string"
fi

Output

The substring is present in string

In this example, we have a string "Hello World" stored in variable string1, and a substring "Hello" stored in variable substring. We use == operator with * wildcard to check if substring is present in string, and if it is, program prints "The substring is present in string" on screen.

Additional Techniques

Apart from techniques mentioned above, there are a few more techniques for string comparison in Bash that you may find useful.

Regular Expression Comparison

Regular expression comparison is used when you need to compare strings based on a pattern. For example −

Example

string1="hello world"
pattern="^hello.*$"
if [[ "$string1" =~ $pattern ]]
then
   echo "The string matches pattern"
fi

Output

The string matches pattern

In this example, we have a string "hello world" stored in variable string1, and a regular expression pattern "^hello.*$" stored in variable pattern. We use =~ operator to check if string matches pattern, and if it does, program prints "The string matches pattern" on screen.

Length Comparison

Length comparison is used when you need to compare strings based on their length. For example −

Example

string1="hello"
string2="world"
if [ "${#string1}" -lt "${#string2}" ]
then
   echo "The first string is shorter than second string"
fi

In this example, we have two strings "hello" and "world" stored in variables string1 and string2. We use ${#string} parameter expansion to get length of each string and then use -lt operator to check if first string is shorter than second string. If it is, program prints "The first string is shorter than second string" on screen.

Null Comparison

Null comparison is used when you need to compare if a variable is set or not. For example −

string1=""
if [ -z "${string1+x}" ]
then
   echo "The variable is not set"
fi

In this example, we have an empty string stored in variable string1. We use ${string+x} parameter expansion to check if variable is set or not and then use -z operator to check if it is not set. If it is not set, program prints "The variable is not set" on screen.

Best Practices for String Comparison in Bash

When working with string comparison in Bash, there are some best practices that you can follow to ensure that your scripts are reliable and efficient. Here are some of best practices that you can consider −

Use Double Quotes

When comparing strings in Bash, it's essential to enclose variables in double quotes to ensure that strings are properly interpreted. For example −

string1="hello"
string2="world"
if [ "$string1" == "$string2" ]
then
   echo "The two strings are equal"
fi

In this example, we use double quotes to enclose variables string1 and string2, ensuring that strings are properly interpreted.

Use [[ ... ]] Instead of [ ... ]

When performing string comparison, it's recommended to use [[ ... ]] conditional expression instead of [ ... ] test command. [[ ... ]] expression provides additional functionality, such as pattern matching and regular expressions. For example −

Example

string1="hello world"
pattern="^hello.*$"
if [[ "$string1" =~ $pattern ]]
then
   echo "The string matches pattern"
fi

Output

The string matches pattern

In this example, we use [[ ... ]] conditional expression to compare string with regular expression pattern.

Conclusion

In conclusion, Bash string comparison is a fundamental concept that every developer should be familiar with. By understanding various comparison operators and techniques, you can write reliable and robust scripts that can handle string comparison effectively. As you continue to work with Bash, you'll encounter different scenarios that require string comparison, and it's essential to be creative and find best approach to handle each scenario. With knowledge you've gained from this article, you're now equipped to handle most string comparison scenarios in Bash.